Symbol mapping method for repetition channel coding

ABSTRACT

A symbol mapping method for repetition coding is disclosed. The symbol mapping method comprises performing repetition coding on codeword to output repeated codeword symbols, and mapping the repeated codeword symbols with subcarriers located in different localized resource blocks. According to the embodiments of the present invention, it is possible to obtain maximum reliability in a receiving side by mapping codeword bits with subcarriers to reduce the number of bits having low reliability when a transmitting side uses repetition coding. Also, it is possible to improve decoding throughput and obtain channel diversity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/754,538, filed on Jan. 30, 2013, currently pending, which is acontinuation of U.S. patent application Ser. No. 12/348,121, filed onJan. 2, 2009, now U.S. Pat. No. 8,391,405, which also claims the benefitof earlier filing date and right of priority to Korean PatentApplication No. 10-2008-0022477, filed on Mar. 11, 2008, and pursuant to35 U.S.C. §119(e), and also claims the benefit of U.S. ProvisionalApplication Ser. No. 61/018,674, filed on Jan. 3, 2008, the contents ofall of which are hereby incorporated by references herein in theirentireties.

FIELD OF THE INVENTION

The present invention relates to a symbol mapping method, and moreparticularly, to a method for mapping codeword bits with subcarriers toreduce the number of bits having low reliability when a receiving sideuses repetition coding.

DISCUSSION OF THE RELATED ART

For wireless communication, channel coding is designed to minimize itsoption considering complexity of a receiving side and a transmittingside. However, in an actual communication system, a code rate lower thanthat of the designed channel coding may be required considering systemcoverage and throughput deterioration in a specific position within acell. In this case, in IEEE 802.16 or 3GPP LTE (Long Term Evolution)system, a repetition coding has been introduced. Particularly, in caseof packet retransmission, effect of repetition coding can naturally beobtained through HARQ operation. However, in case of control channel,packets include repetition coding during initial transmission to obtaineffect of repetition channel.

FIG. 1 briefly illustrates a procedure of conversion of packets to OFDMtransmission signals when channel coding is generally used.

If a source operation is required, a data packet is converted through,for example, an interleaving or ordering scheme (110). The convertedbits become codewords by channel coding (120), and before the codewordsare converted to actual transmission symbols, channel interleaver (130)is used, if necessary, to obtain diversity. Afterwards, the codewordbits are converted to channel symbols in accordance with a transmissionmodulation order through channel symbol modulation (140), and can beallocated from a channel symbol mapper (150) to a subcarrier position.If MIMO (Multi-Input Multi-Output) operation (160) is included in amapping procedure (170) of mapping channel symbols with OFDM symbols,the MIMO operation (160) may be performed before or after the channelsymbols are mapped with subcarriers.

In this case, repetition coding may be performed in the channel codingprocedure (120), or may be performed after the bits are converted to thechannel symbols (140). When repetition coding is performed in a unit ofcodeword bits, saving effect of subcarriers can be obtained if thenumber of codeword bits does not reach a multiple of the number ofchannel modulation bits.

After the channel coding codewords generated as above are received by areceiving side, they are decoded to the original data packet by adecoding algorithm. However, for optimal decoding, all codeword bitsshould have same reliability, or reliability should be distributed inaccordance with a structure of specific channel coding. If not so,suboptimal throughput is obtained. This is applied regardless of thefact that repetition is performed in a bit level or a symbol level.

If repetition coding is used, repetition coding starts with repetitionof original codewords. Repetition coding can be comprised of bit levelrepetition. Bit level repetition can be performed in a source bit typeof FIG. 2 or a codeword bit type of FIG. 3. As illustrated in FIG. 4,after codewords are converted to transmission symbols, a repetitionmethod of the transmission symbols may be used.

In FIG. 2 to FIG. 4, difference in actual throughput is determined bytotal energy used in transmission. If total code rates are uniformlyprovided, similar throughput is obtained regardless of the methods inFIG. 2 to FIG. 4. However, if repetition is performed in a bit level, itis advantageous in that subcarriers can be used more effectively whenthe system transmits a signal.

If repetition coding is used, channel variation of each bit should beminimized so that a receiving side obtains optimal diversity effect.However, in an actual wireless communication channel, channel status isvaried per subcarrier and reliability of codeword bits is varieddepending on gain. Namely, reliability of specific code bits may bedeteriorated by deep fading of channel. Accordingly, the transmittingside should properly set a mapping mode for mapping codeword bits withsubcarriers, considering such channel variation.

However, in case of the current IEEE 802.16 system, although repetitionis defined in a codeword bit level, there is no specific mapping orderafter codeword bits are converted to channel symbols, and repetitionsimply follows a symbol mapping order of the whole system. Particularly,if codewords become a multiple of a specific length, effect according torepetition is simply changed to chase combining, and very poor decodingthroughput is caused.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a symbol mappingmethod for repetition channel coding, which substantially obviates oneor more problems due to limitations and disadvantages of the relatedart.

An object of the present invention is to provide a symbol mapping methodfor repetition channel coding which enables maximum reliability in areceiving side by mapping codeword bits with subcarriers to reduce thenumber of bits having low reliability when a transmitting side usesrepetition coding.

To achieve the object and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, asymbol mapping method according to one embodiment of the presentinvention comprises performing repetition coding on codeword to outputrepeated codeword symbols, and mapping the repeated codeword symbolswith subcarriers located in different localized resource blocks.

In another aspect of the present invention, a symbol mapping methodaccording to another embodiment of the present invention comprisesperforming repetition coding on codeword to output repeated codewordsymbols, and mapping the repeated codeword symbols with subcarrierslocated at a distributed resource block in a manner that a distancebetween the subcarriers can be maximized.

According to the embodiments of the present invention, it is possible toobtain maximum reliability in a receiving side by mapping codeword bitswith subcarriers to reduce the number of bits having low reliabilitywhen a transmitting side uses repetition coding. Also, it is possible toimprove decoding throughput and obtain channel diversity.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a diagram illustrating an example of a method for generatingOFDM symbols according to the related art;

FIG. 2 is a diagram illustrating an example of repetition coding forrepeating source bits;

FIG. 3 is a diagram illustrating an example of repetition coding forrepeating codeword bits;

FIG. 4 is a diagram illustrating an example of repetition coding forrepeating the result modulated using channel symbols;

FIG. 5 is a diagram illustrating a method for generating OFDM symbolsaccording to an embodiment of the present invention when repetitioncoding is performed before channel symbols are generated;

FIG. 6 is a diagram illustrating a method for generating OFDM symbolsaccording to another embodiment of the present invention when repetitioncoding is performed after channel symbols are generated;

FIG. 7 is a diagram illustrating an example of a symbol mapping methodapplied to FIG. 5 and FIG. 6;

FIG. 8 is a diagram illustrating an example of a symbol mapping methodapplied to FIG. 5 and FIG. 6 when localized resource blocks areallocated in a distributed mode; and

FIG. 9 is a diagram illustrating an example of a symbol mapping methodapplied to FIG. 5 and FIG. 6 when a time axis is considered.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. However, it is tobe understood that various modifications can be made in the followingembodiments of the present invention, and the scope of the presentinvention is not limited to the following embodiments.

The embodiments of the present invention can be divided in accordancewith a position at which a repetition can be performed. Examples of theposition at which the repetition can be performed include a case whererepetition is applied in a bit level and a case where repetition isapplied in a symbol level. Also, examples of the position to whichrepetition can be applied include a case where repetition is appliedbefore performing interleaving by an interleaver and a case whererepetition is applied after the interleaving.

However, repetition of source bits or repetition before the interleavingis not proper. Instead, if repetition is performed after theinterleaving in a bit level or symbol level, a mapping rule forobtaining diversity of channel can be made.

Accordingly, for the position at which repetition coding is performed,two methods are considered as follows. The first method is that codewordbits are directly used as illustrated in FIG. 5 to enhance subcarrierutility. The second method is that modulation is applied to codewordbits as illustrated in FIG. 6 to generate channel symbols and thenrepetition is performed.

FIG. 5 illustrates a method for generating OFDM symbols according to oneembodiment of the present invention when repetition is performed beforechannel symbols are generated.

First of all, a data packet is converted through, for example, aninterleaving or ordering. The converted bits are converted to codewordsthrough channel coding by a channel encoder 520. If necessary,interleaving by an interleaver 530 is applied to the codewords to obtaindiversity.

Next, repetition coding (540) is applied to the interleaved codewords.When repetition is performed after the interleaving is performed by theinterleaver 530, channel status can be applied to repetition coding mostpreferably.

Afterwards, channel symbol modulation (550) is performed for each bitgenerated by the repetition coding to generate channel symbols.

A channel symbol mapper 560 allocates the generated channel symbols tocorresponding subcarrier positions. If necessary, the channel symbolmapper 560 performs MIMO operation (570). An additional interleaving canbe performed after the repetition coding in case that the channel symbolmapper 560 is a simple mapper which uses sequential mapping on the timeor frequency axis.

Finally, OFDM symbols are generated through a mapping procedure (580) ofmapping channel symbols with OFDM symbols.

FIG. 6 illustrates a method for generating OFDM symbols according toanother embodiment of the present invention when repetition coding isapplied after channel symbols are generated.

First of all, a data packet is converted through, for example, aninterleaving or ordering (510). The converted bits are converted tocodewords through channel coding by a channel encoding 520. Ifnecessary, interleaving by an interleaver 530 is applied to thecodewords to obtain diversity.

Next, channel symbol modulation (550) is performed for each bit of theinterleaved codewords to generate channel symbols.

Next, repetition coding (540) is applied to the channel symbols.

A channel symbol mapper 560 allocates the generated channel symbols tocorresponding subcarrier positions. If necessary, the channel symbolmapper 560 performs MIMO operation (570). An additional interleaving canbe performed after the repetition coding in case that the channel symbolmapper 560 is a simple mapper which uses sequential mapping on the timeor frequency axis.

Finally, OFDM symbols are generated through a mapping procedure (580) ofmapping channel symbols with OFDM symbols.

In either of FIG. 5 and FIG. 6, repetition throughput can be improvedthrough the channel symbol mapper 560. Namely, a channel symbol mapper560 can be designed so that repeated codewords have different kinds ofdiversity per repetition. The channel symbol mapper 560 can beequivalently designed by being replaced with an interleaver at aprevious stage. Interleaving can be applied to the modulated symbols orrepetition-coded bits.

Generally, in case of an independently distributed channel, signalsreceived through different channels have channel capacity C as expressedin Equation 1.

$\begin{matrix}{C = {\log_{2}\left( {1 + {{SNR}{\sum\limits_{i = 0}^{N_{d} - 1}{h_{i}}^{2}}}} \right)}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In the Equation 1, N_(d) represents the number of used independentchannels, and h_(i) represents gain of a corresponding channel. In thiscase, channel capacity can be regarded for one symbol. A method forobtaining maximum capacity through this channel is to transmit symbolthrough the best channel only. On the other hand, in view of decoding ofcodewords, it is more advantageous for optimal throughput that allcodeword bits have the same capacity, i.e., the same reliability thanthat a specific symbol has high capacity, i.e., high reliability.

Although reliability condition is varied depending on a structure andtype of channel coding, if a structure of corresponding channel codingis decoded depending on bit reliability, it is preferable that allcodeword bits have the same reliability as each other.

Accordingly, the channel symbol mapper 560 of FIG. 5 and FIG. 6 shouldallow all codeword bits to have the same value of Equation 1 ifpossible. This means that all repeated codeword bits/symbols shouldexist over the whole system bandwidth.

FIG. 7 illustrates an example of a symbol mapping method applied to FIG.5 and FIG. 6.

Same codeword bits or symbols should be allocated and arranged at themost constant intervals within the system bandwidth. In this case, whensame symbols/bits exist beyond a coherence band, the maximum value ofEquation 1 can be obtained.

Since the time order has already been mixed by the interleaver, thesymbol mapper allocates symbols/bits of one codeword in the frequencyfirst order in the system band when mapping them.

Although the repeated codewords are interleaved with one another, theyshould be spaced apart from one another at the maximum interval asillustrated in FIG. 7. Also, if all codeword bits/symbols are notallocated to one OFDM symbol, the other bits/symbols are mapped with theother OFDM symbols.

The above method is the most effective when subcarriers to be used inthe whole system bandwidth can be used at a constant interval.

However, in an actual system, subcarrier resources can be allocated in adifferent method not the constant interval. For example, in case of 3GPPLTE, a mode of distributed resources is a distribution type comprised ofa bundle of localized subcarriers. By contrast, in case of a resourceallocation structure of IEEE 802.16, a distributed mode is comprised oflogical channels, which are respectively arranged at a random subcarrierinterval in the whole system bandwidth. Although the 3GPP LTE prescribesthat blocks within the coherence bandwidth are beyond diversity rangefor allocation of subcarriers, the IEEE 802.16 is based on sequencesmapped in the order of non-uniform.

FIG. 8 illustrates an example of a symbol mapping method applied to FIG.5 and FIG. 6 when localized resource blocks are allocated in adistributed mode.

In case of a distributed mode according to the 3GPP LTE, symbol/bitmapping of codewords is varied depending on the number of repetitiontimes. Also, when localized resource blocks 810, 820 are allocated,symbol/bit mapping of codewords is varied depending on whether how manyresource blocks are allocated.

Preferably, same codeword symbols/bits are arranged in differentlocalized resource blocks 810, 820 if possible.

For example, when the number of localized resource blocks 810, 820 is 2and the number of repetition times is 1, since the number of samecodeword bits/symbols is 2, each symbol is mapped with differentlocalized resource blocks 810, 820.

Meanwhile, when the number of repetition times is 2, since the number ofsame symbols/bits is 3, two of three symbols/bits, i.e., S_(i) ⁰ andS_(i) ¹ are located in the same localized resource block. In this case,the symbols S_(i) ⁰ and S_(i) ¹ are arranged to be spaced apart fromeach other within the same localized resource block and then mapped withsubcarriers, so as to obtain the maximum diversity effect.

If the number of repetition times increases, it is preferable that samesymbols/bits are allocated to all localized resource blocks uniformlywithin the limits of the possible and that same symbols are arranged tobe spaced apart from one another within the limits of the possiblewithin the same resource blocks.

Meanwhile, in case of a distribution mode based on the IEEE 802.16,since there is no rule for subcarrier allocation position, optimalsequence or mapping can be searched in accordance with each repetitionthrough full search.

However, since search result may be varied depending on the number oflogical subchannels used in one OFDM symbol and a rule may be varieddepending on the number of a total of OFDM symbols, it is not efficientthat a mapping method is searched through full search. It is preferablethat a mapping rule is used based on the number of a total ofsubcarriers used in one OFDM symbol instead of symbol mapping based onthe frequency first order in a logical subchannel direction. Namely,instead of channel symbol mapping based on the subchannel order,subcarriers within the used subchannel are mixed and same symbols can bearranged to be spaced apart from one another if possible in accordancewith the aforementioned mapping rule. Even if subcarriers are notarranged to be spaced apart from one another at a constant interval, itis preferable that the position of subcarriers, in which same symbolsexist, is determined supposing that the subcarriers are arranged atconstant intervals.

As described above, the mapping method for mapping channel symbolswithin one OFDM symbol has been described.

However, in the actual system, the number of OFDM symbols allocated fortransmission of one packet is defined as more than one. Particularly, itis likely that resource allocation in the 3GPP LTE or future system willbe defined based on TTI which is a transmission unit. In this case, atime axis can also be defined to maximize diversity. Namely, samesymbols exist should be spaced apart from each other on the time axis aswell as the frequency axis if possible. Particularly, like the 3GPP LTE,if a resource allocation type is a localized distributed allocationtype, it is likely that same symbols exist in the same localizedresource blocks as a repetition factor increases. In this case, samesymbols/bits within the codewords arranged within the same localizedresource block or the coherence band can be arranged to be spaced apartfrom each other on the time axis if possible.

FIG. 9 illustrates an example of a symbol mapping method applied to FIG.5 and FIG. 6 when a time axis is considered.

FIG. 9 illustrates several types of repeated symbols when resources areallocated over several OFDM symbols. In FIG. 9, two localized resourceblocks 900, 1000 exist.

If the smallest boxes 901, 902, 903, 910, 920, and 930 correspond to onesubcarrier, in one OFDM symbol, repeated symbols can be arranged to bemost spaced apart along the frequency axis, and the other repeatedsymbols can be arranged in the other time position.

For symbols 901-903 transmitted at the same frequency bandwidth, it ispreferable that the symbols are not arranged to be adjacent to eachanother (901, 902) on the time axis but arranged at constant intervalson the time axis (902, 903).

Also, for maximum effect, it may be considered that the repeated symbolsare arranged to be most spaced apart from each other (910, 920).Moreover, in one OFDM symbol, the other repeated symbols can be arrangedin another localized resource block 1000, whereby the minimum number ofsymbols/bits within the codewords can be obtained. At this time, if thetime interval between the repeated symbols is sufficiently great, eventhough the repeated symbols are arranged in the same resource, effect isnot reduced. Accordingly, the symbols are arranged in differentlocalized resources if possible or beyond the coherence band.

The present invention relates to a method for mapping codeword bits withsubcarriers so that the number of bits having low reliability is reducedwhen a transmitting side uses repetition coding. The present inventioncan be applied to a base station and a mobile station, which constitutea system such as 3GPP LTE system and IEEE 802.16 system, etc.

It will be apparent to those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

What is claimed is:
 1. A method for transmitting a signal at a basestation in a mobile communication system, the method comprising:performing repetition coding on an information bit to generate aplurality of information bits; obtaining a plurality of modulationsymbols from the plurality of information bits; mapping the plurality ofmodulation symbols to one or more orthogonal frequency divisionmultiplexing (OFDM) symbols; and transmitting the plurality of mappedmodulation symbols, wherein the plurality of modulation symbols aremapped separately from each other in a frequency domain when theplurality of modulation symbols are mapped to one OFDM symbol, andwherein at least two of the plurality of modulation symbols are mappedseparately from each other in the frequency domain to a same OFDM symbolwhen the plurality of modulation symbols are mapped to two OFDM symbols.2. The method of claim 1, wherein the plurality of mapped modulationsymbols are transmitted via a control channel.